1. Field of the Invention
The present invention relates generally to B channel communication over the Integrated Services Digital Network, and more particularly to a transceiver system for communicating data bytes from one computer to another that accommodates byte alignment throughout the data path from a host computer through an Integrated Services Digital Network to a destination computer, and eliminates the need for conventional circuitry for ordering data in High Level Data Link Control (HDLC) frames required in existing systems for processing by routers in the communications network.
2. Description of the Prior Art
Historically, data has been transferred between computers almost from their invention. If the distance to be traversed is large in comparison to the size of the computer, the data is typically converted in format to a representation designed to make efficient use of the communications medium. For example, digital data is processed for transmission over voice lines by modems which convert ones and zeros into frequencies in the range of the sounds for which analog telephone systems have been optimized. The advent of digital telephone has changed both the engineering and economic aspects of data communications. Specifically, many hardware requirements are now met with reliable and low cost digital integrated circuits. When hardware is expensive, most systems are designed to maximize the use of each hardware element in order to minimize costs. Thus, for example, early computers were generally serial shift register based, that is, a one bit wide data stream was repeatedly shifted through one bit wide circuits. As hardware costs fell, hardware was replicated, so that four or eight bits were operated on at once, with significant increase in operational speed. As hardware costs fell further, sixteen bits were handled in parallel while today's popular computers handle thirty-two and even sixty-four bits in parallel.
While computer systems have evolved from one bit through 4, 8, 16, 32 and 64 bit operations, communications systems are largely still stuck at the one bit level. The reason for this is better understood by reference to FIG. 1 which illustrates various communications network interconnections wherein data terminal equipment (DTE) can communicate through an Integrated Services Digital Network (ISDN) 10. Different systems of controlling access are used depending on whether the communication is controlled by what is known as point to point protocol or by one of the various LAN systems such as Ethernet, ARCnet or Token-Ring symbolically illustrated by 12, 14 and 16 in FIG. 1.. A DTE within a Token-Ring system 16, for example, can not be directly interconnected with a DTE in an ARCnet LAN. In order to accomplish communication between the different LAN systems as well as non LAN terminals such as DTE 18, devices called routers 20, 22 are used to direct communication and translate the various system protocols from one form to another depending on the particular source and destination types. The routers 20, 22 require incoming data to be in a special form called a High Level Data Link Control (HDLC) frame 24 as shown in FIG. 2. The data, placed in the information field 26 block is surrounded by organizational bits including Flags 28, 30 to indicate the beginning and end of each frame. The digital structure of a Flag as indicated at 32 is defined as a byte (8 bits) including a zero followed by six ones and then another zero. This creates a problem in the information field 26 because such a series of data bits would be erroneously read as a Flag. In order to avoid this error, all such sequences of bits in the data field are modified. This is done by a special HDLC circuit that stuffs a zero after every detected fifth one of a series, in effect expanding an 8 bit byte such as 34 to nine bits as shown at 36. This processing effectively destroys the byte alignment provided by a source DTE which must be restored before a destination DTE can receive it. This processing involves the expense of the HDLC circuitry and is aesthetically displeasing and organizationally complicating due to the destruction of the byte alignment from the computer.
FIG. 3 shows a simplified block diagram of a system wherein a source 38 is coupled through an HDLC circuit 40 to an ISDN communications channel 42 through a second HDLC 44 to a destination computer 46. The ISDN network is internally byte aligned, transmitting data in groups of 8 bits. The organization of a byte aligned ISDN frame is shown in FIG. 4. Each frame 50 is comprised of 48 bits including two 8 bit bytes each for two channels B1 and B2, 52-58. Also shown are four D channel bits 60, and blocks 62 containing standard organizational bits. The HDLC circuit as explained above destroys the byte alignment, but if there were a system that could maintain byte alignment throughout, the HDLC circuit as above explained could be eliminated, resulting in a more orderly and less costly system. Clearly, there is a need for a system that could handle the data transmission without the HDLC byte misalignment.